The art of surveying involves the determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. In order to make these measurements, a surveying instrument frequently comprises an electronic distance measuring device (EDM) which may be integrated in a so-called total station. A distance measuring total station combines electronic, optical and computer techniques and is furthermore provided with a computer or control unit with writable information for controlling the measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail.
Further, when performing distance measuring or surveying tasks using a distance measuring total station at a work site, it is often desirable to measure a surface or volume of an object being present on the work site. In such a work site, it may, for example, often be desirable to scan a surface of an object, for example, a wall of a building to obtain an image of the wall. For such applications, a distance measuring total station may be implemented as a geodetic scanner for determining the appearance of the object or target based on the measurements of distances to positions of interest at the surface of the target. Such a scanner may register the surface or volume of the target or even monitor changes in a scene.
In a conventional EDM, a light beam is emitted as a light pulse toward the surface of a target (or scene) and the light beam that is reflected against the surface is detected at the EDM, thereby generating a signal. Processing of the detected signal enables the determination of the distance to the surface, i.e. the distance between the EDM and the target. In a conventional geodetic scanner, the light beam is guided over each one of a number of positions of interest at the surface of the target using a beam steering function. A light pulse is emitted toward each one of the positions of interest and the light pulse that is reflected from each one of these positions is detected in order to determine the distance to each one of these positions. However, the detected signal representative of the reflected light beam (or light pulse), i.e. the return signal, may have a wide dynamic range. In other words, the strength or power of the return signal may vary significantly from one position to another. Variations of the return signal may be explained by e.g. differences of reflectivity between different positions at the surface of the target and/or large differences in the topography of the target. As a result, distances determined from a return signal having a too large or too low power are not accurate because of difficulties in handling a wide dynamic range at the measuring device (scanner). The detected signal may e.g. be saturated or subject to too much noise.
In a first alternative, the measurements for which the strength of the return signal is above a first threshold or below a second threshold may be considered as invalid and therefore deleted. However, such an alternative is not desirable since the appearance of the target object is determined from a limited number of valid measurements only, i.e. with a reduced resolution. Further, this method implies unnecessary processing of invalid measurements.
In a second alternative, a conventional method is to stop the beam steering function of the scanner at every position of interest at the surface of the target and perform a two-step measurement for each one of the positions of interest. In a first step or measurement period, a first light pulse is transmitted toward the target, and the reflected light pulse is detected and processed to calculate an appropriate gain or gain value. Typically, if the power representative of the detected light pulse is considered to be low, i.e. below a predetermined threshold, the gain is set at a value larger than 1. On the other hand, if the power representative of the detected light pulse is considered to be large, i.e. above a predetermined threshold, the gain is set at a value lower than 1. Then, in a second step or measurement period, a second light pulse is sent toward the target and the reflected light pulse is detected and amplified using the calculated gain. The amplified signal is then processed for determining the distance to the target. As a result, the distance is measured with an appropriate gain for each one of the positions of interest. However, a drawback of such a method and scanner is the limited measurement rate, and thereby rather low overall efficiency.
Thus, there is a need for providing new methods and systems that would overcome these problems.